Re: List of multiple elements

• To: mathgroup at smc.vnet.net
• Subject: [mg112054] Re: List of multiple elements
• From: Raffy <adraffy at gmail.com>
• Date: Fri, 27 Aug 2010 04:07:51 -0400 (EDT)
• References: <i55gq0\$2cu\$1@smc.vnet.net>

```On Aug 26, 3:48 am, "Dr. Wolfgang Hintze" <w... at snafu.de> wrote:
> Given the list
>
> a={1,1,2,1,2};
>
> we can reduce multiple instances of elements using
>
> b=Union[a]
> {1,2}
>
> The question is now how to compute the list of multiple elements.
> In our example this would be m={1,1,2}.
>
> A possible solution is
>
> m[x_]:= Flatten[Take[#, {1, Length[#] - 1}] & /@ Select[Split[x],
> Length[#] > 1 &]]
>
> m[a]
> {1,1,2}
>
> I'm sure there is a much more elegant solution. Can you suggest one?
>
> Remark
>
> m[a] can be viewed as a kind of difference: m[a] = a "MINUS" Union[a]
> Inverting gives the "SUM": a = m[a] "PLUS" Union[a] =
> Join[m[a],Union[a]]
>
> Regards,
> Wolfgang

v = {1, 1, 2, 1, 2};

Join @@ (Gather[v][[All, 2 ;;]])

Join @@ (ConstantArray[#1, #2 - 1] & @@@ Tally[v])

```

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